"Humour (or humor in American English) is the tendency of particular cognitive experiences to provoke laughter and provide amusement. The term derives from the humoral medicine of the ancient Greeks, which taught that the balance of fluids in the human body, known as humours (Latin: humor, "body fluid"), controlled human health and emotion." From Wikipedia....
That's exactly why I said "I don't get it". Admitting the joke went over my head. That's not hard to understand, and this thrust of the conversation has already been covered.